A general wireless communication system transmits/receives data through one downlink (DL) band and through one uplink (UL) band corresponding to the DL band (in the case of frequency division duplex (FDD) mode), or divides a prescribed radio frame into a UL time unit and a DL time unit in the time domain and transmits/receives data through the UL/DL time unit (in the case of time division duplex (TDD) mode). A base station (BS) and a user equipment (UE) transmit and receive data and/or control information scheduled on a prescribed time unit basis, i.e. on a subframe basis. The data is transmitted and received through a data region configured in a UL/DL subframe and the control information is transmitted and received through a control region configured in the UL/DL subframe. To this end, various physical channels carrying radio signals are formed in the UL/DL subframe.
Meanwhile, to use a wider frequency band in recent wireless communication systems, introduction of carrier aggregation (or bandwidth aggregation) technology that uses a wider UL/DL bandwidth by aggregating a plurality of UL/DL frequency blocks has been discussed.
FIG. 1 illustrates an example of performing communication in a multicarrier environment.
A multicarrier system or carrier aggregation (CA) system refers to a system for supporting a wide bandwidth by aggregating a plurality of carriers each having a narrower bandwidth than a target bandwidth. A CA system is different from an orthogonal frequency division multiplexing (OFDM) system in that DL or UL communication is performed using a plurality of carrier frequencies, whereas the OFDM system carries a base frequency band divided into a plurality of orthogonal subcarriers on a single carrier frequency to perform DL or UL communication. When a plurality of carriers each having a narrower bandwidth than a target bandwidth is aggregated, the bandwidth of each of the aggregated carriers may be limited to a bandwidth used in a legacy system in order to ensure backward compatibility with the legacy system. For example, the legacy LTE system supports bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz and an LTE-Advanced (LTE-A) system evolved from the LTE system may support a bandwidth wider than 20 MHz using only bandwidths supported in the LTE system. Alternatively, CA may be supported by defining a new bandwidth irrespective of the bandwidths used in the legacy system. The term multicarrier is used interchangeably with the term CA or bandwidth aggregation. Contiguous CA and non-contiguous CA are collectively referred to as CA. For reference, when only one component carrier (CC) is used for communication in TDD or when only one UL CC and one DL CC are used for communication in FDD, this corresponds to single carrier communication (non-CA).